One of the common elements required in electrical circuit devices is the simple pullup (or pulldown device) from an active device to one of the power supply buses. The pullup is simple if used to construct a circuit using discrete components in that all that is required is selecting a resistor of the desired resistance and tolerance, connecting it between an active device, such as an open collector transistor, and V.sub.cc and the transistor's output would be pulled up to V.sub.cc once the transistor is forward biased. With the advent of the integrated circuit (IC) however, fabricating a resistance onto a wafer substrate, such as silicon or gallium arsenide, takes special consideration particularly when resistivity and tolerances play an important part in circuit operation.
For example, as SRAMs have evolved from the small 4 Kb memory arrays to more densely packed array sizes, tolerances of pullup resistances (or pullup loads) had to be tightly controlled. In order to minimize standby current many fabrication processes adopted using an active device as the pullup. In CMOS fabrication it is common to see a PMOS transistor acting an active load device, by providing a current path between a memory cell access transistor and the power supply bus. In this manner the PMOS transistor could be gated on only when the desired line was to be pulled to V.sub.cc and turned off otherwise, thereby virtually eliminating leakage current and minimizing standby current for the SRAM device as a whole.
Conventional methods to fabricate active pullup devices are expensive, such as is the method of fabricating a PMOS pullup by the use of selective epitaxially grown silicon (EPI). Using EPI requires a tightly controlled process method that requires two levels of polysilicon and two interconnects to form the PMOS transistor. As the SRAM generation continues to grow, cost becomes an even more critical factor that must be taken into account in order to produce competitively priced parts.
The present invention introduces a method to fabricate an active pullup P-channel device fabricated on top of an SRAM's NMOS storage cell transistor thereby using a common gate electrode between the two devices. This fabrication technique provides for a less expensive method to form an active load than do conventional fabrication processes.